As it stands, I'm thinking of getting rid of my Fuji/Nikon system for Sony. Problem is, I can't rent nor try out the camera anywhere, so I have to resort to asking. How is the AF in dimly lit situations? Like say reception, or anything else with low/no light? Does it lock on to subject, or are you staring mostly at a really sharp wall behind the subject?

This is a make it or brake it deal for me. The reason I want the Sony, is that I would like to have just one system. Right now I do personal stuff with Fuji (small size, great IQ) and weddings with D600 (all around awesome, except when it comes to focusing in backlit situations. It's crappish.) Sony would be light enough to carry around, and IQ would be sufficient for pro-work. Win-win!

But it really needs to nail the AF. If it's as good as D600, fine. That camera has it's gimmicks but it's good enough. I am not expecting 1DX/D4 type of performance.

Anyone with practical experience? Gripes? Epiphanys? I really don't care about the tracking. I use S-AF anyway 99% of the time. But I need it to nail it. It seems that 55/1.8 24-70/4 would be good combos.

First, background in focus is a CDAF problem, usually helped by bright shapes in the background and dark subjects.

PDAF is more assured, but requires a certain amount of brightness - either some ambient light or faster lenses. Faster lenses, used wise open, limit DOF which makes AF more critical.

The A7 does reasonable well with the FE35 and FE55 in low light, but I would not recommend the FE2870 for low light. On the A7S however, the same FE2870 is very surefooted, even though you are used CDAF (but with increased DOF).

The A7R is most tricky - it needs the faster (prime) lenses, but also has only CDAF - but now with shallow DOF.

I would recommend to consider using the A7 with the fast prime lenses, and the A7S with a zoom lens, and use a fast prime on the A7S for some (B&W) candle light mood shots.